The present invention relates to an apparatus and a method for generating a two-dimensional representation of an objection portion arbitrarily arranged within an object, as is employed, e.g., for processing X-ray image data in industrial quality control of products.
The technical field of application of the invention described here comprises processing of X-ray image data, in particular in industrial quality control of products, which is performed by means of X-radiation. One example of the most important cases of application mentioned here is the detection of so-called pipes or porosities in castings such as aluminum wheels. The particular difficulty lies in the fact that the X-ray image data is fraught with image noise as a matter of the underlying principle, said image noise negatively influencing (automatic) recognition of the casting defects. To reduce the image noise, the image content is therefore typically averaged over several pictures of the same kind. In addition to X-ray image technology, the present invention generally relates to imaging methods such as, e.g., ultrasound methods, radar imaging, etc.
In the production of castings, fabrication defects such as air bubbles, porosities or cracks may occur, which may significantly influence mechanical properties and, thus, reliability. Therefore, reliable testing of such parts is indispensible particularly for safety-critical parts in the field of automobiles or aeronautics. In extreme cases, such defects may cause, e.g., a breakage of spokes in aluminum wheels when stress is applied, or during further processing (machining) may cause optical impairments which eventually give cause for items being discarded, which for economic reasons should occur as early in the value creation chain as possible.
Since such production defects are concealed within the material, in the industrial area, pictures are created, mainly by means of X-ray technology, of the relevant areas of the castings wherein cavities are reflected by variations in brightness due to the reduced absorption of the X-rays as compared to the surroundings. Mostly, the variations in brightness in these image data are then processed automatically, detected by suitable software, and then possibly result in the device under test being discarded.
One problem arising in this context is the inevitable noise occurring in the pictures. The variations in brightness caused by porosities are, particularly for minor porosities, within the range of the variations in brightness caused by the noise, so that a reduction of the image noise significantly simplifies future further processing of the image data. Since the image noise directly depends on the number of impinging X-ray quanta, averaging is generally performed over a number of similar pictures or over a relatively long exposure time so as to thereby reduce the noise. By averaging, those image parts which are constant within the images are accentuated, and at the same time the variable image parts (i.e. the noise) are reduced. In order to achieve this it may be useful to keep the device under test still so as to avoid defects caused by shaking.
Since the castings are typically larger than the X-ray sensor available, testing is performed in different steps, each step comprising two-dimensional imaging of part of the device under test on a detector/sensor. To this end, either the X-ray source/detector or the device under test (or both at the same time) are moved to image the next section under test on the detector. Since it is irrelevant for the X-ray images whether the new perspective has been created by moving the device under test or the picture-taking unit or both, it shall be assumed below that only the device under test is moved.
Since, as has already been mentioned, the device under test is to be still while the sequence of pictures to be averaged is taken, a test system typically performs a sequence of picture-taking/movement cycles. In each of these cycles the device under test is accelerated, and is decelerated as soon as the next test position is reached, so as to then take a further sequence of pictures. The acceleration (and the deceleration) are delimited by the handling system, since, e.g., an aluminum wheel which is decelerated abruptly will simply slip through within a gripping device if the contact pressure is not sufficient. In addition, one may wait until potential mechanical vibrations caused by the deceleration process have subsided, so as to avoid the blurring caused by movements before one may start taking the sequence of pictures.
The duration of taking the sequence of pictures depends on the desired measure of noise reduction, but typically ranges from about 10 to 16 pictures, which leads to a reduction of the noise by a factor of 4. Longer durations would indeed further reduce the noise, but as a rule are not practicable since testing of a part is to be performed within as short a time as possible.
In an aluminum wheel to be tested e.g. in a total of 30 pictures taken, the acceleration/deceleration of the wheel typically accounts for 500 ms per image, and the integration of 16 pictures at 25 complete images per second takes a further 16/25 s=640 ms. This results in a testing time of about 35 seconds for the entire wheel.
The main disadvantage of conventional technology therefore is the large amount of time needed for repeatedly accelerating and decelerating the device under test.
In addition, with such images taken, conventional technology has been limited, by definition, to the two-dimensional projection of a three-dimensional object, so that no evaluation of the depth information within the single image can occur. In addition, it is possible that in one perspective, material defects are masked by other image parts in the projection, so that it may be useful to project each location of the device under test from several perspectives so as to detect these maskings in at least one of several perspectives.
In addition, tracking of potential material defects in noisy, non-averaged single images, and algorithmic discarding of artifacts by means of various defect properties has been known.
Conventional technology further describes tangential radiography, which is described, e.g., in DE 695 22 751 T2. Also, purely two-dimensional methods referred to as (digital) laminography have been known. In this context, a camera takes pictures of an object from various angles so as to therefrom image a planar face in a focused manner, and to image objects located outside this plane in a blurred manner.
In Jing Liu et al., “Generalized Tomosynthesis for Focusing on an Arbitrary Surface”, in IEEE Transactions on Medical Imaging, Vol. 8, No. 2, June 1989, a tomosynthesis method is described with which a reconstruction algorithm composes, starting from different tomograms, composes image points within an image plane, the image points of individual tomograms being superimposed such that those image points within an image section of interest constructively will superimpose, whereas such points which are not located within the image section will superimpose at random. In this context, a general concept is disclosed which also enables composing three-dimensional structures on the basis of the individual tomograms.
EP 1 225 444 A2 discloses an X-ray means for creating laminagrams of an object to be examined by means of tomosynthesis. In this context, an X-ray source is located above an object to be examined, which in turn is located above an X-ray detector. The object to be examined is situated on a table which is displaceably mounted. For this reason, a number of single images may be taken which represent shifted image sections of the object to be examined. For superimposing the individual partial images into one overall image, the displacements of the table, which may be detected by means of potentiometers, for example, are used.
In Thomas D. Kampp, “The backprojection method applied to classical tomography”, in Med. Phys. 13 (3), May/June 1986, a projection method is described which enables creating split images on the basis of individual tomograms, a split image corresponding to a plane which comprises six degrees of freedom, i.e. three translational and three rotational ones. The individual tomograms may be generated using classical tomographs, the object to be examined being able to undergo various motions, such as linear, circular or rotational motions.